1. Field of the Invention
The invention relates to an electron beam source, a method of operating an electron beam source and an electron optical apparatus using such beam source.
An electron optical apparatus, such as an electron microscope and an electron lithography apparatus using electrons for imaging purposes, comprises at least one electron beam source for providing an electron beam which is used in the apparatus for electron optical imaging or other purposes.
2. Description of the Related Art
Typical demands which an electron beam source should fulfill are the provision of an electron beam having properties such as a high beam current, a high brightness, an intensity which is sufficiently constant over time, and a low width of a distribution of kinetic energies of the electrons in the beam. The width of such distribution is often referred to as FWHM (full width at half maximum). Often it is desirable to operate the electron beam source under conditions which do not permit obtaining a particularly high vacuum.
The conventional electron beam source comprises a cathode body having a source surface from which the electrons emanate, and an anode disposed at a distance from the source surface for providing an electrical extraction field for supporting the emission of electrons from the source surface. A heater may be provided for heating the source surface to further assist the emission process of the electrons from the surface.
Depending on a strength of the extraction field and the temperature of the source surface, plural physical processes may be identified which cause emission of the electrons from the source surface. These processes are illustrated in e.g. Reimer, Scanning Electron Microscopy:Physics of Image Formation and Microanalysis 2nd edition, Springer series in optical sciences, 1998). To leave the source surface the electron has to traverse a potential barrier at the metal-vacuum interface, which potential barrier is referred to as work function xcfx86w or as chemical potential xcexce.
In a thermionic emission process, the temperature of the source surface is high enough such that the electrons from the Fermi level EF of the cathode material can overcome the potential barrier by thermionic excitation. For example, thermionic emission is achieved at temperatures of the cathode material above 2500 K to 3000 K using a cathode made of tungsten.
At the low temperatures such that thermionic excitation does not substantially contribute to electron emission and at high electrical excitation fields, a field emission process is the dominating process in electron emission from the surface. Electron sources operating in such regime are referred to as field emission sources. Field emission from a tungsten tip having a radius of about 0.1 xcexcm starts when the electrical field strength at the surface is 107 V/cm or higher. Such high fields decrease the width of the potential barrier in front of the source surface to a few nanometers so that electrons from the Fermi level EF can penetrate the potential barrier by a wave mechanical tunneling effect.
The conventional electron beam source further comprises a Schottky emission gun in which the potential barrier or the work function xcfx86w is decreased by the Schottky effect. The electrical extraction field in the Schottky emission source is about ten times lower, as compared to the field emission gun, such that a sufficient narrowing of the potential barrier allowing a substantial contribution of the wave mechanical tunneling effect to the total emission does not occur. The Schottky emission source is heated to a temperature which is substantially lower than the operating temperature of a corresponding thermionic emission source. However, the temperature is sufficiently high that the electrons may overcome the remaining potential barrier which is reduced by the Schottky effect.
In view of a low energy width (FWHM) of the electron source, the source surface should be at a low temperature to avoid a thermal broadening of the energy width. From this point of view the field emission source is preferred since this type of source may be operated at room temperature. As a drawback, the field emission source requires operation at ultra high vacuum conditions for preventing destruction of the source surface by ion bombardment. The field emission source is also insufficient with respect to a maximum beam current.
Schottky emission sources are often used as a compromise between low temperatures of the source surface in view of a low energy width, and avoiding making high demands in terms of vacuum conditions. A drawback of the Schottky emission source is a reduced stability of the beam current. Small changes in operating conditions, such as changes of temperature and surface contamination, already result in comparatively high changes of the beam current.
The conventional electron beam source further comprises a photo emission source as illustrated in e.g. U.S. Pat. Nos. 4,460,831 and 5,808,309. In the photo emission source, the source surface is illuminated with a photon beam for releasing electrons from the source surface by a photo effect. The photo emission source is used in applications where the electron beam has to be rapidly switched on and off. Rapidly switchable light sources are readily available, and the electron beam intensity immediately follows in time with the switched photon intensity. However, the photo effect requires using radiation of a particularly short wavelength in photo emission sources using source surfaces made of typical materials employed as electron sources. The energy of the photons incident on the source surface must be higher than the potential barrier or the work function xcfx86w. Light sources of sufficiently short wavelength are expensive and complicated to operate.
From U.S. Pat. No. 5,041,724 there is known a rapidly switchable photo emission source in which the photon energy necessary for generating photo emission is reduced by reducing the height of the potential barrier by applying an additional strong electrical extraction field, resulting in field assisted photo emission, or by heating the source surface, resulting in thermally assisted photo emission.
Further, U.S. Pat. No. 5,763,880 discloses reducing the potential barrier or work function xcfx86W of a cathode body by applying an oxide or nitride layer to the source surface.
As illustrated above, electron sources having a reduced operating temperature lack adjustability of the beam intensity due to an increased contribution of the wave mechanical tunneling effect.
Accordingly, it is an object of the present invention to provide an electron beam source operated at a reduced temperature of the source surface while allowing for an improved adjustability of a desired beam intensity.
Further, it is an object of the present invention to provide an electron optical apparatus, in particular an electron microscope, generating an electron beam having a reduced energy width and an improved adjustability of the beam current.
It is a further object of the present invention to provide a corresponding method of operating an electron source.
The invention provides an electron beam source for generating a beam of electrons wherein an intensity of a photon beam incident on a source surface for emission of electrons is adjusted dependant on an intensity of the generated electron beam, and wherein heating of the source surface by some process different from the illumination with the photon beam assists in releasing electrons from the source surface.
According to an embodiment, the electron beam source comprises a cathode body having a source surface for emitting electrons, and an anode disposed at a distance from the cathode for generating an electrical extraction field. The extraction field is provided to assist the electrons in overcoming the potential barrier, i.e. to decrease the potential barrier at least by some amount as illustrated above with respect to the Schottky emission source, and by reducing a width of the potential barrier by at least some amount such that the wave mechanical tunneling effect may provide at least some contribution to the electron emission, as illustrated above with respect to the field emission process.
The electron beam source further comprises a photon source for generating at least one photon beam directed to the source surface for assisting in the electron emission, as illustrated above with respect to the photo effect. Thus, at least the extraction field and the photon beam contribute together to release electrons from the source surface.
In view of a reduced intensity of the photon beam, and a reduced energy of the photons or increased wavelength of the light of the photon beam, a heater is provided for heating the cathode body such that thermionic excitation of the electrons also contributes to the emission thereof from the source surface.
Therein it is possible to operate the electron beam source at operating conditions close to that of the Schottky emission type source, such that the thermionic excitation process substantially contributes to the electron emission process. The electron beam source generates a substantial electron beam different from a dark current also in a situation in which the photon beam is not directed to the source surface. A large amount of the energy necessary for emitting the electrons may then be provided by the heater which is of a simple configuration, as compared to the photon source. The photon source may than be used mainly to adjust the intensity of the electron beam, and in particular to maintain the intensity of the electron beam at a constant level.
Even when an energy deposition of the photons in the cathode body may result in heating the source surface above room temperature, such that the heating of the cathode body due to the photon beam also generates some thermal contribution to the emission process; such contribution is low compared to the contribution of the heater. A maximum intensity of the photon beam in a normal mode of operation of the electron source is advantageously limited, such that the source surface, starting from room temperature, will not reach a temperature above 1700 K, or above 1200 K or advantageously not above 700 K, if only the photon beam is incident on the source surface, and no other type of energy, such as by an additional heater, would be actively supplied to the cathode body.
Even though the photon beam provides a relatively low contribution to heating the cathode body, the photon beam provides an important means of adjusting the intensity of the emitted photon beam, since the intensity of the electron beam is adjusted by changing the intensity of the photon beam. For this purpose, the electron beam source further comprises a detector for detecting a beam current of the electron beam and for generating a measuring signal representative of the beam current. The electron beam source further comprises a controller responsive to the measuring signal and configured for controlling the photon source in the normal mode of operation of the electron beam source to change the intensity of the photon beam based on the measuring signal.
Hereby the property of the electron beam source which is to be adjusted, i.e. the intensity of the electron beam, is used as the measured quantity for adjusting the photon beam as the regulated quantity, for finally controlling the electron beam intensity as desired. According to a preferred embodiment the controller may be configured for maintaining the electron beam intensity at a constant level. Alternatively, the controller may be configured to control the electron beam intensity in view of a desired integral beam current, i.e. a total amount of charge emitted by the electron beam source. It is then possible to compensate drifts and changes in the electron beam source which might arise if the intensity of the photon beam is not controlled dependant on the electron beam intensity. Such drifts and changes may result from small changes in temperature of the source surface or from contamination of the source surface. Such changes have a particularly high effect in situations where the temperature of the source surface is low in the normal mode of operation.
According to a further embodiment, the material from which the source surface is made, the electrical extraction field, the heating of the cathode body, and the intensity of the photon beam, are adjusted relative to each other in the normal mode of operation of the electron beam source, such that in a situation where the photon beam is not directed to the source surface the beam current is more than about 30%, in particular more than about 65%, and advantageously more than about 80% of the beam current in the normal mode of operation of the electron beam source in which the photon beam is directed to the source surface.
According to a further embodiment the heater comprises an electric heater such that an Ohmic resistance generates the heat for heating the cathode body.
According to a further embodiment the detector detects the electron beam intensity by measuring an intensity of electrons incident on a beam stop, i.e. by measuring a current supplied by the beam stop. The beam stop may comprise a beam stop having an aperture traversed by the electron beam, and in particular the anode for generating the extraction field.
According to a further embodiment, the source surface of the cathode body comprises barium oxide, or is made of barium oxide, since this type of material has a particularly low work function xcfx86W. According to an advantageous embodiment, the barium oxide is applied to a cathode body made of e.g. tungsten.
According to a further embodiment of the invention the photon beam is used for a further purpose apart from its function in view of adjusting the electron beam intensity in the normal mode of operation. In an annealing mode of operation, the photon beam is directed to the source surface for annealing the source surface and the portion of the cathode body providing the source surface. In the normal mode of operation changes in a configuration of the source surface may occur, such as deposition of ions of a residual gas in a vacuum chamber in which the electron beam source is disposed. Such changes may be reversed by annealing the source surface by increasing the intensity of the photon beam such that the temperature of the source surface in the annealing mode of operation exceeds the temperature of the source surface in the normal mode of operation by more than about 100 K, preferably more than about 200 K, and in particular more than about 300 K or about 500 K.
According to a further embodiment the electron source is used in an electron optical apparatus such as an electron microscope and an electron lithography apparatus.